[AF_RXRPC]: Key facility changes for AF_RXRPC

Export the keyring key type definition and document its availability.

Add alternative types into the key's type_data union to make it more useful.
Not all users necessarily want to use it as a list_head (AF_RXRPC doesn't, for
example), so make it clear that it can be used in other ways.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/key.h

@@ -160,6 +160,8 @@ struct key {
 	 */
 	union {
 		struct list_head	link;
+		unsigned long		x[2];
+		void			*p[2];
 	} type_data;
 
 	/* key data